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Quaternary International 159 (2007) 32–46

Climatic imprints in Quaternary valley fill deposits of the middleTeesta valley, Sikkim Himalaya

Lukram I. Meeteia,b, Sanjaya K. Pattanayaka, Arun Bhaskara,Maharaj K. Pandita,c, Sampat K. Tandonb,�

aCentre for Interdisciplinary Studies of Mountain and Hill Environment, Academic Research Centre Building, Patel Marg, University of Delhi,

Delhi 110007, IndiabDepartment of Geology, University of Delhi, Delhi 110007, India

cUniversity Scholars Programme, BLK ADM, Level 6, National University of Singapore, 10, Kent Ridge Crescent, Singapore 119260, Singapore

Abstract

Quaternary alluvial sediments occur as distinct terrace and fan deposits in the middle Teesta valley in the belt between the Main

Central Thrust and the Main Boundary Thrust in the Sikkim Himalaya. These sequences are characterized by lithofacies deposited by

braided river channels, debris flows and hyperconcentrated flows. The channel flow deposits constitute relatively well sorted, well

imbricated and clast-supported gravels with coarse to medium sand matrix. Mostly poorly sorted, weakly imbricated to disorganized

matrix supported pebble to boulder gravels with silty sand represent debris flow deposits. Hyperconcentrated flow deposits consist of

clast-supported, poorly developed sorted polymodal gravel facies with poorly developed imbricated fabric, and generally occupy the

lower parts of the terrace and fan sequences. The alternation from hyperconcentrated flow to channel flow deposits is predominant in the

sequence, and is possibly the response to different climate modes. The high discharge and supply of sediments as well as the dispersal and

deposition of these materials in the trunk stream is attributed to millennial–multimillennial climatic perturbations during the Quaternary.

Climate change has a dominant role in the valley aggradation and incision cycle.

r 2006 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction

Quaternary valley fill deposits in the mountain valleysarchive the changes in valley form and stratigraphicarchitecture induced by perturbations in the tectonic-climate system. This implies that a causal relationshipbetween Quaternary climate change and river behavior canbe established by studying the exposed key sections inQuaternary landforms preserved in intermontane valleys.However, tectonic and anthropogenic factors also drive,and influence, fluvial development (Vandenberghe, 2002).It is challenging to decouple the climatic and tectonicinfluences.

The Quaternary uplift in the Himalaya varies from a fewup to 2000 and more metres (Iwata, 1987; Selby, 1988). Theaverage uplift rate is estimated to be of the order of

e front matter r 2006 Elsevier Ltd and INQUA. All rights re

aint.2006.08.018

ing author.

ess: [email protected] (S.K. Tandon).

1–2mm/year or even more (Iwata, 1987). Different studies(Harrison et al., 1992; Lave and Avouac, 2001; Zeitleret al., 2001; Hodges et al., 2004) have indicated that theHimalaya has experienced an accelerated uplift in the past.However, perturbations in the climate on multimillennial-scale (Goodbred, 2003) can have a strong influence onincision history of the fluvial system, and therefore thecalculated uplift rate may be different from the real upliftrate.In the Himalaya, some recent studies in the Ganges river

system have suggested a link between climate andsedimentation based upon (i) lithofacies characters ofalluvial deposits in Kali Gandaki valley in Nepal (Moneckeet al., 2001), (ii) temporal coincidence of sedimentation andclimatic events in Marsyandi valley in Nepal (Pratt-Sitaulaet al., 2003), (iii) aggradation and incision phases in thenorthern and southern tributaries of Ganga (Goodbred,2003) and (iv) correlation of river flood plain behaviourwith intensity of South Indian Monsoon based on age

served.

dx.doi.org/10.1016/j.quaint.2006.08.018

mailto:[email protected]

ARTICLE IN PRESSL.I. Meetei et al. / Quaternary International 159 (2007) 32–46 33

dates below the interfluve surface from Kanpur and Kalpi(Gibling et al., 2005). The Ganga river system is consideredimportant for such investigations because it is governed bya single climatic-forcing, the South-Asian monsoon. More-over, in Sikkim and Darjeeling Himalaya, the Teesta riversystem, now draining into the Brahmaputra River, was atributary of Ganga until 1787 (Fergusson, 1863; Mukul,2000). Therefore, the sediment archives in the Teestafluvio-sedimentary system could have been sensitive toclimate forcing associated with the South Asian Summermonsoon.

The valley-fill deposits of the Teesta river basin inSikkim Himalaya offer an opportunity to understand theclimate and tectonic imprints on them. Studying the terracesequence along ca. 65 km stretch in the Teesta basin inSikkim Himalaya from Mankha in the moutainous regionto Sevoke in the piedmont region, Sinha Roy (1980)concluded that the glacial and periglacial landforms occurin the north-central part of the valley and abundant fluvialterraces occur in the lower stretches. He correlated theterrace tiers on the basis of relative elevation from thethalweg, and on the degree of weathering and soildevelopment in the terrace deposits. This study indicatesthat between the Main Central Thrust (MCT) in the northand Main Boundary Thrust (MBT) in the south a three-tierterrace system in the Teesta basin is preserved. Theselandforms occur at several places between tributaryconfluences where the trunk valley is relatively wide.However, careful examination of this work indicates that(i) at Mankha the middle terrace is not preserved, (ii) atSingtam the middle and lower terraces are not preserved,(iii) at south Rangpo the middle terrace is absent, and (iv)at Deorali the top terrace is not preserved. Moreover, apreliminary basin wide reconnaissance survey indicatedthat the preserved three-tier terrace system at Sirwani isdisturbed by H.E. Power project development and theterrace sections are now covered with debris derived fromthe powerhouse cavern. However, at Mangalbare, a three-tier terrace is well preserved (Fig. 1), and as evident fromthe longitudinal section given in Sinha Roy (1980) thisregion lies upstream of a knickpoint which correspondswith the E–W fault to the south of Rabangla. Therefore,this terrace system has been examined along with theassociated tributary fan deposits for the purpose ofunderstanding climate/tectonic influences on their evolu-tion and development.

2. Background

With the refinement of knowledge on Quaternaryclimate and its history, the role of climate change insedimentary processes and sequence development acrosscontinental margins has been better understood in recentyears (Posamentier and Allen, 1993; Blum and Tornqvist,2000; Goodbred, 2003). This development has begun toimpact the strongly held view that tectonics has a largercontrol on catchment evolution and fluvial response

(Goodbred, 2003). The paucity of studies of the fieldsequences in the context of the response of depositionalprocesses to climate change has remained a limitation.However, recent publications (Posamentier and Allen,1993; Blum and Tornqvist, 2000) have emphasised theinterplay between climate and eustasy.River hydrology and sediment production in many

subtropical (monsoon) systems seem to co-vary withglacioeustasy (Goodbred, 2003). Molnar and England(1990) believe that the reported tectonic uplift in themountain regions for the late Cenozoic is not the realuplift. They opine that deep climate driven incision haslead to higher mountain peaks (i.e. apparent uplift) but anoverall decrease in mean elevation through isostasy. Such alink of climate impacts with tectonic processes is consistentwith high sediment fluxes and rapid rates of incision duringthe Quaternary (Curray and Moore, 1971; Burbank et al.,1996; Einsele et al., 1996; Leland et al., 1998). Since theshort-term atmospheric circulation is comparatively robustunder the dominance of the Himalaya-Tibetan plateau, theSouth Asian monsoon varies less at centennial timescalescompared to the millennial scale (COHMAP, 1988;Sirocko et al., 1993; Luckge et al., 2001). The runoff fromthis South-Asian monsoon wholly drives the Ganges riversystem (Goodbred, 2003). The Teesta river system, a majortributary of Ganga in the historical past, must therefore bedominantly controlled by this single climatic forcing.The sediment dispersal processes in the Ganges river

system were different at different time windows during theQuaternary. The last interstade (�MIS-3) 58–24 ka wascharacterized by a moderate climate that was perhaps onlyslightly cooler and drier than the present (Goodbred, 2003and the references therein). During this period, the riverdischarge was moderate and lower compared to the presentand the glacial erosion processes probably producedprodigious volumes of sediments. With the inception oflast stadial period (LGM) 24–18 ka, low insolation andstrong glacial boundary conditions weakened the summermonsoon and significantly reduced regional precipitation.The LGM is characterized by (i) widespread aridityrevealed from several proxy records and modeling results(Van Campo, 1986; COHMAP, 1988), (ii) significantdecrease in river discharge into the Bay of Bengal asindicated from paleosalinity (d18O) (Cullen, 1981; Du-plessy, 1982). Prell and Kutzbach (1987) suggested a 25%decrease in regional precipitation. During this cold regimeit is likely that most glacial sediments (till and debris)remained in the production zone and were not transporteddownstream.Following the LGM, increased insolation strengthened

the summer monsoon (COHMAP, 1988; Gasse et al.,1991). Greater precipitation as evident from modelreconstructions and pollen, lake and surface-ocean records(Cullen, 1981; Swain et al., 1983; Van Campo, 1986; Dill etal., 2003) has been interpreted in the past �12 ka. TheHolocene hypsithermal, being driven by peak regionalinsolation (�9 ka) and weakening glacial boundary

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Fig. 1. Location map of the study area showing the South Tibetan Detachment System (STDS), Main Central Thrust (MCT) and Main Boundary Thrust

(MBT). Dotted areas represent the major glacier complexes. The drainage and glaciers were compiled from Survey of India Toposheets and the tectonic

features were taken from Acharya (1989) and Dasgupta et al. (2004).

L.I. Meetei et al. / Quaternary International 159 (2007) 32–4634

conditions, was perhaps a period of highest precipitation; a35% increase in monsoon precipitation from the LGM tohypsithermal is suggested by Prell and Kutzbach (1987).After the hypsithermal phase, the climate shifted to aperiod of moderate precipitation. In the Ganges basin, theprecipitation levels remained relatively near to the presentlevel, and probably a slight aridification occurred between2 and 5 ka (Bryson and Swain, 1981; Sirocko et al., 1993;Dill et al., 2003).

3. Study area and geomorphology

The study area is located around Mangalbare village inSouth Sikkim in the zone lying between the MCT and theMBT (Fig. 1). In this region, Quaternary terraces aredeveloped on both the banks of the Teesta River. The

terraces on the west bank are preserved between two fansdeveloped at the confluences of two eastward flowing smalltributaries—Rangpo Khola in the north and Ben Khola inthe south.Originating from Pahunri (or Teesta Kangse) glacier that

lies above 7068ma.s.l., the Teesta river flows southwardthrough gorges and rapids in the Sikkim Himalaya. Threeprominent knickpoints have been observed along theTeesta river profiles, corresponding to the zones of tectonicdiscontinuities, the important ones being the MCT andMBT (Seeber and Gornitz, 1983). In the north, thecatchment of the Teesta basin is mostly glaciated terrain.The terraces and floodplains, valley-side slopes and land-slide slopes, alluvial cones of different generations, kettleshaped depressions, sickle-shaped ranges, leveled plains,undulating plains and deeply dissected valleys, glacial and

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Fig. 2. Geomorphology of the study area. (a) Panchromatic data, (b) LISS–PAN merged data, (c) Geomorphic map of the area prepared from PAN and

LISS–PAN merged data.

L.I. Meetei et al. / Quaternary International 159 (2007) 32–46 35

periglacial deposits are some of the geomorphologicalfeatures observed in the Teesta River basin in Sikkim(Mukhopadhyay, 1982). The landforms and their assem-blages in the Teesta river basin and its innumerabletributaries are the result of climatic processes, tectonicdeformation, denudation and deposition. Five distinct geo-climatic zones viz. (i) frigid zone (above 4000m a.s.l.) withglacial, periglacial and fluvio-glacial processes, (ii) coldzone (between 2500 and 4000m a.s.l.) with periglacial,fluvio-glacial and fluvial processes, (iii) cold temperate zone(between 2000 and 2500m a.s.l.) with fluvio-glacial andfluvial processes, (iv) warm temperate zone (between 1000and 2000m a.s.l.) with fluvial processes and (v) subtropicalzone (up to 1000m a.s.l.) characterise the Teesta Rivercatchment in Sikkim and Darjeeling Himalaya (Mukho-padhyay, 1982). The present day climatic regime of theMangalbare area lies within the milieu of warm temperateand subtropical zones.

High mountain ranges in the Sikkim Himalaya, particu-larly in the north, northwestern and northeastern parts ofthe basin, are covered with snowfields or glaciers (Fig. 1).

The glaciers of Sikkim occur as compound glaciers termedas glacier complexes where a number of glaciers originatefrom a common permanent ice covered region. Five glaciercomplexes, namely Chhombo, Yumthang, Langpo, Zemuand Talung, are present in the Sikkim Himalaya (DPR,2005). The glaciated upper Teesta basin is characterized byhuge accumulation of debris in the form of debris cones,rock-glaciers and alluvial fans, and debris avalanches.Debris is transported mainly in the monsoon season andduring snow-melt period. The middle and lower parts ofthe basin are marked by relatively subdued relief and sloperelated slides and slips.Fig. 2 shows the geomorphological units of the area and

the surrounding terrain. Landslide cones, and depositionallandforms viz. terraces, fan lobes and channel bars arepresent in the region. The terraces are three-tier, and occurat three different altitudinal levels. The upper fan lobe andthe middle terrace occur at the same altitudinal level. Thisterrace is spatially juxtaposed between two upper fan lobes,one along Rangpo Khola in the north and the other alongthe Ben Khola in the south (Fig. 2). In this study, following

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Fig. 3. Morphology of landforms in part of Teesta valley near the confluence of Rangpo Khola. (a) Photograph showing the terrace and fan morphology,

(b) sketch of the confluence of Rangpo Khola at the right bank of Teesta river, (c) schematic cross-section across the Teesta River at Mangalbare village

showing different Terrace levels.


the conventional method of defining depositional land-forms, the present day Teesta channel deposit, the lowerlevel terrace, mid-level terrace and upper-level terrace aredesignated as T0, T1, T2 and T3, respectively (Fig. 3).Similarly, the present day channel deposits of tributarystreams Ben Khola and Rangpo Khola and lower andupper fan lobes have been designated as F0, F1 and F2,respectively.

4. Lithofacies analysis

4.1. Methods used

Detailed mapping has been carried out along the Teestariver channel between Mangalbare in the north and Amloiin the south using PAN and LISS-PAN merged datafollowed by field checking (Fig. 4). The Survey of India1:50,000 topographic maps with 40m contour intervalswere used as base maps. Terrace and fan sequences presentnear the confluences of Ben Khola and Rangpo Khola werestudied for their lithofacies assemblages. The surfaces ofdifferent landform units viz. terrace and fan lobes weremapped (Fig. 4) using Leica-GS5 GPS. The elevation

characteristics of the landforms were measured using GPS.Lithofacies characters and their lateral extensions wereexamined in the field and recorded in the form ofdescriptive sketches and logs supported by photographs.The thickness of each facies unit, matrix and clast ratio,and lithology of clasts were recorded. The thickness oflithounits in well developed sections has been used toapproximate the volume of different facies units, expressedas percent of the total volume of landform.

4.2. Landforms and lithofacies

4.2.1. Morphology of landforms

The spatial arrangement of different landform units inthe study area with respect to Rangpo Khola, Ben Kholaand Teesta river is shown in Figs. 2 and 4. Fig. 5 representsa schematic section showing the landforms (along line AB,Fig. 4) along the west bank of the Teesta. The geometricparameters of different landform units are given in Table 1.The terrace T3 occupies the highest altitudinal position

(385–400m a.s.l.) in this region and its thickness variesfrom 20 to 22m. It is unpaired and only present at the westbank of the Teesta River (Fig. 4). The terrace T2 is paired

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Fig. 4. Spatial disposition of different geomorphic units in the vicinity of Mangalbare village. These units were mapped using GPS. The points numbered

1–11 represent the locations of lithologs. A–B is the traverse along which the schematic cross-section (see Fig. 5) was prepared.

Fig. 5. Schematic cross-section through Quaternary deposits along A–B traverse (see Fig. 4) at the Ben Khola–Teesta and Rangpo Khola–Teesta

confluences illustrating the geomorphic and stratigraphic relationships. Since T1 occurs only at the east bank of Teesta, it is not shown in this section.


and occurs between 364 and 368m a.s.l. Its surface isinclined at 1–61 towards the SE-E and 51 towards the NE.The terrace T1 is present at the east bank of Teesta,

between 357 and 359m a.s.l. Two distinct fan lobes, F2 andF1, present at both the tributary confluences are incised;the incision varies from 0.5 to 7m. The terrace T2 and fan

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Table 1

Distribution and geometrical parameters of different landforms

Geomorphic

Units

Location Area cover (m2) Surface

inclination

Elevation above

sea level (m)

Incision (m) Total number of

facies present

Fan lobes at the Rangpo Khola confluence

Fan lobe F2 North bank 52,049 21 towards E 364–383 2–7 4

South bank 12,047 31 towards E 368–375 3–5

Fan lobe F1 North bank 32,696 51 towards E 361–390 2–4 5


Terraces at the banks of Teesta river

Terrace T3 West bank 24,487 21 towards E 385–400 25–30 3

Terrace T2 West bank 1,15,711 Vary from 1 to 61

towards SE–E to

51 towards NE

364–368 10–12 5

East Bank 1,57,310 21 towards E 364–359 8–10

Terrace T1 East Bank 58,967 11 towards E 357–359 2–4 6

Fan lobes at the Ben Khola confluence

Fan lobe F2 North bank 10,320 11 towards N 351–376 1–4 2


Fan lobe F1 North bank 17,806 41 towards NE 346–369 0.5–2 5

South bank 17,458 21 towards E 345–371 0.5–3

Elevation of the present day water level of the Teesta river at the study area varying from 345 to 355m during the field survey.

Table 2

Lighofacies of Terraces and Fan lobes at Mangalbare

Facies Texture Sedimentary Structures Bed contacts Bed thickness Interpretation

Clast supported gravels

A Clast supported

boulder gravel

Poorly sorted

subangular clasts. Clast

size range from 25 to

70 cm. At places cobbles

occur with the boulders.

Matrix consists of coarse

sand and amounts to

20–40%

Disorganised pattern of

clast distribution. Locally

poorly imbricated clasts

with a(p) b(i) fabric. The

clast size increases towards

the bottom

Partly erosive Bed thickness varies

from 60 cm to 4m in

the Terrace T2. In F1

fan lobe it is �20 cm

Hyperconcentrated

flow (Channel floor

with flash flood)

B Clast supported

boulder cobble

pebble, granule

gravel

Subrounded to

elongated rectangular

shaped, at places platy

clasts. In T2 the clast size

ranges from 20 to 30 cm.

Optimal clast size is

50 cm. In Fan lobes F1

and F2 the clast size

ranges from 10 to 80 cm

with optimal size of

1.3m. The matrix

consists of coarse sand,

silt and pebbles and

amounts to 20–45%

Well-imbricated

horizontally stratified unit

with imbrications at places

showing a(t) b(i) or a(p)

b(i) fabric. Accumulation

of particles on the obstacle

clast’s (boulder’s) stoss

side and accretion of

grains in its downstream

wake is observed at places

in F2 and T2. Platy clasts

show imbrications with

a(p) b(i) fabric. In some

units in F1 and F2, crude

irregular cross-

stratification is seen.

Inverse to normal grading

with pseudo-laminations is

present in F1

Erosive Bed thickness varies

from 1 to 4m in

Terrace T2 and Fan

lobe F2 and 20 cm to

1m in F1 and T1

Channel flow

(Braided and

Transverse bars

Terrace and channel

lag deposits)

C Unsorted clast

supported cobble

and boulder gravel

Angular to subrounded

and platy clasts with size

range 15–60 cm.

Maximum clast size is

100 cm in F2. Matrix

consists of coarse sand

and fine gravels and

amounts to 15–40%

Imbricated large clasts

show a(t) b(i) fabric in F1.

Smaller clasts in the matrix

are disorganized to a(p)

fabrics


from 18 to 80 cm in

Fan lobe F1. It is

80 cm in Fan lobe F2

Hyperconcentrated

flow (Braided bars

of high flood)


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Table 2 (continued )

Facies Texture Sedimentary Structures Bed contacts Bed thickness Interpretation

Matrix supported gravels

D Matrix supported

cobble and boulder

gravel

Subrounded to well-

rounded clasts range in

size from 2 to 40 cm.

Maximum clast size is

50 cm in F1 and 80 cm in

F2. In some sections

subrounded to sub-

angular clasts range in

size from 2 to 25 cm.

Matrix consists of coarse

to medium sand and at

places fine pebbles and

amounts to 60–80%

Moderate to well-sorted,

imbricated with a(p) b(i)

fabric. Occasional

megaclasts with a(t) b(i)

fabric. Finer clasts are

disorganized. Cross-

stratification is observed in

T2. Accumulation of

particles on the obstacle

clast’s (boulder’s) stoss

side and accretion of

grains in its downstream

wake is observed in F1.

Crude horizontal

stratification and normal

to inverse grading present

in F2


in different

landform units: T3

(35 cm), T2 (90 cm),

F1 (50–60 cm), F2

(1–1.7m)

Channel flow

(fluidal sediment

flow in Longitudinal

and braided bar

environment)

E Matrix supported

boulder to cobble

and pebble gravel

with clays

Sub-rounded to

rectangular clasts with

mean clast size 40 cm

and maximum clast size

is up to 60 cm.In thin

sequence (ca. 30 cm)

subrounded to platy

clasts range in size from

5 to 7 cm.Matrix consists

of mud, coarse sand and

fine gravels with

substantive amounts of

clays (ca. 60%)

Poorly sorted ungraded

clasts. The proportion of

matrix increases upward

Erosive Bed thickness is

1.75m with 1.5m

thick soil capping in

terrace T2 and is

about 50 cm in F1

and 4m in F2

Debris Flow

(Proximal braided

fan)

Sandy facies

F Sandy pebble and

granular gravel

Very angular, poorly

sorted few pebbles with

granules embedded in

poorly sorted angular

medium to very coarse

sands. Amount of

granules is 40%

Stratified poorly

imbricated except few

pebble with a(p) b(t)

fabric. Inversely graded.

Coarsening upward

sequence with reducing

clasts concentration; weak

internal grading

Partly erosive

contact and sharp

upper contact with

thin stratified sandy

unit

Bed thickness 1.5m Hyperconcentrated

flow

G Massive sand Fine to coarse sand with

few feldspar clasts (5 cm)

Graded bed with thin silt

bands

Unknown (?) Bed thickness 1.6m

preserved only in T3

Hyperconcentrated

flow

H Massive silty sand Medium to coarse sand

and silt, occasionally

with fine gravel (10%)

Light grey coloured sand

with ferruginous interlayer

in T1. Horizontal

stratification seen in F2.

The unit is capped with

soil in F1 and F2.


from 30 cm to 1m

Channel flow (point

bars and flood bank)

I Sandy cobble

gravels

Subrounded to

elongated clasts. Matrix

composed of coarse sand

and amounts to 40%

Imbricated cobbles with

a(t) b(p) fabric. stratified

and cohesive layer are

present.


from 30 to 35 cm

Channel flow

(longitudinal bar

head)


lobe F2 occur at almost similar altitudinal levels. The fan lobeF1 is inset into the fan lobe F2. The fan lobe surfaces aremostly inclined towards east at 11 to as high as 51. At places,the fan lobes are covered with landslide cones (Fig. 5).

4.2.2. Lithofacies and depositional flows

Nine distinct facies types are recorded from theselandform units (Table 2). These nine facies units are

grouped into three assemblages: (i) clast supportedgravels, (ii) matrix supported gravels, and (iii) sandy facies(Table 2).

4.2.2.1. Clast supported Gravel Facies. The major part ofterrace T2 and fan lobes developed at both the Ben Kholaand Rangpo Khola confluences are built up of clastsupported gravels. The clast content varies from 60% to

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Fig. 6. Lithologs of the fan lobes and terraces present around Mangalbare. Thick unit boundaries indicate erosional contacts. A ¼ clast supported

boulder gravel, B ¼ clast supported imbricated boulder, cobble, pebble and granule gravel, C ¼ unsorted clast supported cobble and boulder gravel,

D ¼ matrix supported boulder, cobble and pebble gravel, E ¼ matrix supported boulder cobble and pebble gravel with clays, F ¼ sandy granule gravel,

G ¼ massive sand, H ¼ massive silty sand, I ¼ sandy cobble gravels.


85% in these facies units. The clasts mainly consist ofphyllite and quartzite. However, in the east bank, granitegneiss clasts predominate in terrace T2. Three distinct faciestypes (A to C, Table 2) are recognized in the clast-supported gravels of which the facies A and C areinterpreted as hyperconcentrated flow deposits, and faciesB is interpreted as a channel deposit.

Facies A occurs at the base of terrace T2 and fan lobe F1

at both the tributary stream confluences (Fig. 6). Clast sizeranges from 25 to 70 cm. Disorganised to poorly developeda(p) b(i) fabric and clast supported nature together suggestthat this facies was deposited by hyperconcentrated flows.Facies C differs from facies A by its clast size (15–60 cm)with dominance of cobbles and by poor a(t) b(i) fabric.This facies is 20 cm to 1m thick and occurs at the base offan lobe F1 at Ben Khola and fan lobe F2 at RangpoKhola. These deposits are interpreted to have formed by ahyperconcentrated flow due to rapid flow expansion andtractional freezing as described elsewhere in alluvial fansettings (Benvenuti and Martini, 2002).

Facies B is different from the facies A and C because ofits strong imbrication and/or stratification and representsdeposition from stream flows. Facies B is present in all thestudied sections and its thickness varies from 20 cm to 4m(Fig. 6). This facies occurs over a strong erosive base. Thisfacies is interpreted as braided bar and channel lagdeposits. There is marked textural variation within thisfacies in different sections. Horizontal stratification witha(p) or a(t) fabric as well as presence of obstacle clasts withlee and stoss side deposition in terrace T2 and fan lobe F1

and F2 of both the tributary fans indicate that these wereformed under turbulent rapid flows (Teisseyre, 1977). Atthe base of the facies unit in fan lobe F2 at Rangpo Kholaconfluence and in the middle of fan lobe F1 at Ben Kholaconfluence, and terrace T2 (Fig. 6) the presence ofelongated clasts with crude irregular cross stratification

(Fig. 7f) indicate that these were laid down by fluidalsediment flow in a transverse and braided stream environ-ment (Lawson, 1982; Nemec and Steel, 1984). At the top offan lobe F1 of Ben Khola as well as in the middle of fanlobe F2 of Rangpo Khola and Terrace T2, inverse-normalgrading with pseudolaminations are present (Fig. 7g). Theclasts in these parts show a(t) b(i) fabric. These featuresindicate that this facies was formed by the higher-energyflows which passed through a slope-break zone whereturbulence developed in the upper part of the flow.However, original dispersive pressure still dominated inthe lower part (Nemec and Steel, 1984).

4.2.2.2. Matrix supported gravel facies. These faciesoccur at different stratigraphic levels within various land-forms. There are two different types of facies named D andE (Table 2). D is interpreted to be deposited by channelflows and E by debris flows (facies G and J). The debrisflow deposits are dominant in fan lobes, particularly in F2

fan lobe at Ben Khola confluence. The channel flow faciesare predominant in the upper part of terrace T3 and T1,and in the middle part of terrace T2.The thickness of the matrix supported gravel facies D of

channel flow origin vary from 10 cm to as high as 1.7m(Table 2). It is preserved in all the studied landforms exceptin the F2 fan lobe of Ben Khola with its confinementtowards the upper part of the sequence. At the top ofsections 6, 9 and 11 (Fig. 6) this facies is characterized bymatrix-supported cobble and boulder gravels with indivi-dual boulders reaching a diameter of 0.8m. The presence ofwell rounded cobbles and boulders with a(p), b(i) fabricand cross-stratification indicate that they represent fluidalsediment flows, deposited under turbulent stream floodcondition (Nemec and Steel, 1984). Thick deposits of thisfacies are preserved in terrace T2 (litholog 8) and fan lobeF2 (lithologs 4 and 11) of Rangpo Khola and Ben Khola

ARTICLE IN PRESS

Fig. 7. Photographs of terraces and present day Teesta River channel showing different facies (a) channel bar deposits at the east bank of Teesta, (b)

channel gravel bar showing the imbricated megagravel (as defined in Blair and McPherson, 1999), (c) Terrace T1 at the east bank of Teesta, (d) Terrace T2

section at the right bank of Teesta river, (e) Terrace T3 (oldest Terrace)at the west bank of Teesta (f) imbricated crude thick cross strata of cobble pebble

gravels of middle part of terrace T2 and (g) clast supported pebble gravels with inverse-normal grading. The length of the scale and pen shown in (a) are 30

and 15 cm. The height of the person in (b) is 1.7m. The length of each blue and white unit in (c) is 50 cm. The length of hammer in d–f and g is 31 cm.


(Fig. 6) which are characterized by horizontally stratifiedmatrix-supported cobble, pebble and granule gravel. Thesedeposits usually show a(p), b(i) fabric, and occasionally fewmegaclasts show a(t), b(i) fabric. These are interpreted asdeposits of longitudinal bars. Such deposits are describedfrom modern and ancient shallow, braided rivers (Hein andWalker, 1977; Nemec and Postma, 1993; Monecke et al.,2001). The sandy channel fills (Fig. 6, litholog 7) at the topof this facies are interpreted as the filling of smallabandoned channels at bar margins. The massive andungraded nature, poor sorting, muddy matrix and taperedgeometry at the margins of the deposited facies E suggestthat these were emplaced by debris flows (Johnson, 1970,1984). In fan lobes F2 and F1 at the Benkhola confluence,the pervasive parallel-to-bedding clast alignment suggeststhat the debris flow experienced full laminar shear beforedeposition (Fisher, 1971; Enos, 1977). The reworked top ofthis facies unit in fan lobes F2 and F1 at both the tributarystream confluences with massive sand gravels and erosivebases suggests that the runoff following the debris flowdeposition caused extensive reworking of the debris flowdeposits (Costa, 1984; Blair and McPherson, 1992;Monecke et al., 2001).

4.2.2.3. Sandy facies. The sandy facies are observed in allthe terraces and both the fan lobes (F1 and F2) at RangpoKhola. The thickest sandy facies G occurs at the base of

terrace T3. Fine to coarse sands with several thinferruginous layers define this unit. This lower unit interrace T3 is characterized by decimeter thick stratification.The unit lacks angle-of repose cross-bedding and internaltruncation surfaces. Such deposits have been observed inmany catastrophic flood-flow deposits and are attributed torapid accumulation under a swift, heavily sediment ladenturbulent flow (Harrison and Fritz, 1982; Pierson andScott, 1985; Smith, 1986; Best, 1992; Sohn et al., 1999).Therefore, this thick sandy unit at the base of T3 isinterpreted to have formed by hyperconcentrated floodflow. The centimeter-thick stratification in this facies mighthave also been a result of repetitive deposition from thin,short-lived traction carpets developed beneath a turbulentflow which carried abundant sediment load and fluctuatedin the suspended-load fallout rate (Hiscott, 1994; Sohn,1997).A different sandy facies unit characterized as granular

sands and sandy unit with pebble and granule (facies F,Table 2) occur in terrace T1 (litholog 5, Fig. 6). Theangularity and ill-sorted nature of the clasts in this faciesindicate that these sediments were also laid down underhyperconcentrated flow condition. Other sandy flow facies(H and I, Table 2) occur at the base of terrace T1 as well asat the top of terrace T2 and Fan lobes F1 and F2 of RangpoKhola (Fig. 6). The centimeter thick horizontal stratifica-tion and imbrication indicate that they probably were

ARTICLE IN PRESSL.I. Meetei et al. / Quaternary International 159 (2007) 32–4642

deposited by channel flows associated with longitudinalbars.

4.2.2.4. Erosional Contacts. All the clast supported grav-el facies are marked by erosional bed contacts, the mostsignificant being 0.5m deep scours at the base of clastsupported imbricated boulder gravel unit (Table 2 in faciesB) in terrace T2 and fan lobe F2 (Fig. 6). Similar erosivebases are associated with the matrix supported gravel units.A prominent example of an erosive base is at the bottom offacies D in terrace T2 (Fig. 6). In the case of the sandyfacies, the base of the facies G (Fig. 6, litholog 9) is notexposed and the base of the facies F (Fig. 6, litholog 5) isgradational. The other contacts are erosive or partlyerosive.

4.2.2.5. Present day channel deposits. The present dayTeesta channel T0 consists of imbricated megagravels ofquartzites and granite gneiss. The term megagravel hasrecently been introduced for sedimentary texture classifica-tion by Blair and McPherson (1999) and represents gravelsin which clast diameter exceeds 4.1m. The quartzite clastsin T0 are angular (Fig. 7b) and the granite gneiss clasts arewell rounded. Units of cross-bedded sands (Fig. 7a) arecommonly observed in the side bars. The size of the clastsand their disorganized pattern indicate that they weredeposited by a hyperconcentrated flow. On the other hand,the cross-bedded sand facies indicate deposition fromchannel flow in a side bar environment. On the F0 surface,

T0 F0

T3

F2

F1

Terrace and Fan Sequence Sedimentation Process Erosion

and Channel FlowHyperconcentrated Flow

Turbulent HeavilySediment Laden Flow

Strong ErosioReworking of

1

Strong ErosionReworking of T

Strong ErosionReworking of

T2

HF

CFEC

HF

EC

CF

EC

CF

ECDF

ECCF

EC

CF

HF

ECCF

HF

T1

Strong Eros

Minor Erosi

Minor Erosi

Minor Erosi

Minor Erosi

Minor Eros

Laminar CohesiveDebris Flow

Channel Flows in Fan andTerrace andCohesive Debris flow inTerrace

Turbulent to LaminarHyperconcentrated Flow

Short lived Traction carpetbeneath a Turbulent Flow

Catastrophic Flood Flow(Hyperconcentrated Flow)

CFEC

CF

HF

ECCF

ECDF

EC

CF

HF

EC

CF

EC

HF

CF

EC

ECCF

CF

Channel Flow

in braided stream Environment

Fig. 8. Sequential evolution of climatic and tectonic events and its relationshi

h ¼ high, s ¼ strong, ¼ moderate; HF—hyperconcentrated flow, DF—debris

in both the streams, Rangpo Khola and Ben Khola,imbricated angular clasts of quartzite are predominant andrange in size from small pebbles to medium boulders. Thesedeposits are channel flow deposits.

5. Vertical stacking of lithofacies

The stacking of different flow deposits is schematicallyshown in Fig. 8 and also indicated in Fig. 5. Thehyperconcentrated flow deposits occur at the base ofterraces (T3, T2) and fan lobes (F1, F2). In terrace T1, theyoccur in the middle and top parts of the landform. Even inthe modern day channel fills (To), hyperconcentrated flowdeposits composed of megagravels are present at places(Fig. 7b) and sandy deposits laid down by channel flowsoccur in the side bars (Fig. 7a) along both the banks ofTeesta River. Some debris flow deposits occur in terrace T2

and fan lobe F1. In the terrace T1, the channel flow depositsoccupy the base.These depositional units are marked by erosive bases;

and the tops probably indicate periods of strong erosionand reworking. The preserved facies in the landformsindicate the beginning of deposition in a braided streamenvironment with high discharge and sediment flux,possibly resulting from glacier melting and related flows.Subsequent to this, periods of intense alluviation corre-spond to intense monsoon conditions with extreme runoffevents and landslide activities. These landslide activities

Water Discharge (W)& Sediment Supply (S)

Climatic andTectonic(?) Events

n andF2

2

andand F

andT2

W=mS=m

W=sS=h

W=sS=h

IntensemonsoonWith extremerunoff events

IntenseMonsoonRainfall

IntenseMonsoonRainfall

IntenseMeltingof Glaciers

Intensewith uplift andLandslidesin pockets

Intensewith uplift andLandslidesin pocketsLow

Moderatewith landslidesin pockets

Low

W=m to sS=m to h

ion

on

on?

on

on

ion

and intensemonsoon withextreme runoffevents

PossibleAge

Present

LateHolocene

LateHolocene(Sitaula et al.,2003)

Last Interstade(Goodbred,2003)

Mid-lateHolocene(Goodbred,2003)

p with sedimentation process. W ¼ water discharge, S ¼ sediment supply,

flow, CF—channel flow; C—erosional contact.


may be associated with seismotectonic events and /or heavyprecipitation.

6. Discussion and conclusion

Climatic change is a fundamental process, which canresult in remarkable variations in sediment supply andwater discharge (Blum, 1993; Vandenberghe, 1993, 2002;Vandenberghe et al., 1994; Pratt-Sitaula et al., 2003;Starkel, 2003). The other competing process is tectonics,including diverse aspects such as source-area uplift, valley-slope failure and vertical fault movement. Related aspectsinclude fluvial avulsion and course adjustments, and baselevel change (Schumm, 1993; Goodbred, 2003).

The hyperconcentrated deposits at the bottom of theterrace T3 occupying a large part of the sequence (Figs. 6and 8) possibly formed during high stream discharge withlarge sediment load. The rounded pebbles, cobbles andboulders of augen granite gneiss, present at the top of T3

(Fig. 7e) suggest that these materials have suffered longdistance transport from the Higher Himalayas. Similardeposits are also present at higher altitudes in the upstreamtract of the studied region at Mankha and Lachung(Fig. 1). The high water discharges with greater sedimentflux are believed to be associated with glacial/deglacialprocesses (Boothroyd and Ashley, 1975; Nemec andPostma, 1993; Vandenberghe et al., 1994). The depositsforming terrace T3, therefore, appear to have been formedin a highly mobile, braided river environment where thelower hyperconcentrated flow deposit formed during theoutburst of floods due to heavy monsoon rain or higherwater discharge due to snowmelt. The upper channel flowdeposit in this landform formed during warmer climatecharacterized by relatively low sediment flux and waterdischarge.

On the basis of finer sediment texture for the major partof T3 and its valley wide preservation in the Teesta River,we speculate that these deposits probably formed duringthe last interstade (58–24 ka.) in which the Ganga megafanswere deposited. During this interstade (MIS-3) Himalayanglaciers extended �10 km beyond their present positions,and locally more than 40 km (Owen et al., 2002). It isprobable that in this period of glacial advancement,prodigious volumes of sediment produced in the upperreaches were transported, leading to massive aggradationin the lower reaches of the tributaries of Ganga. Thelithofacies characters of T3 indicate that probably theTeesta valley was filled with 420m of sandy fill in thisperiod. At this time the valley bottom in the studied regionwas at 370m a.s.l. and the valley was filled up to 400ma.s.l. The formation of such a thick basin-wide depositcould be attributed to strongly seasonal, pseudo-flashy,monsoon discharge with large suspended load and bedload(Schumm, 1977). The sandy deposit of T3 was capped byboulder deposit just before it was incised. This perhapssuggests a shift to an abrupt wetter phase that first causedhill slope failure, locally introducing boulders, and finally

reached a stream power sufficient to incise the boulder lagand underlying T3 sand, apparently scouring the bedrockbefore undergoing a second phase of aggradation. Anincision on the scale of 20–30m is probable after thedeposition of T3 and the valley floor in the region waslowered to 350–360m a.s.l.(Table 1). This incision prob-ably took place in the hypsithermal phase (18–7 ka) whenthere was a brief glacial advance (?). In a similar geologicalsetting, in the Marsyandi river valley in Nepal(1200–1800m a.s.l.), Pratt et al.(2002) are of the view thatcold and arid LGM climate (�24–10 ka) helped generateregolith that remained in situ due to the limited carryingcapacity of the river. Later continued humid conditionswith high discharge removed the material downstream andincised the valley upto the bedrock in a few hundredyears. In the Kali Gandaki valley in west Nepal, Moneckeet al. (2001) described a sequence of thick braidedstream deposits overlain by massive debris flows, nowlocally incised to the bedrock level. For these deposits,Goodbred (2003) postulated that the basal fluvial sequencewas probably emplaced during an interstade and theoverlying debris-flow resulted due to precipitation-induced slope failures at �8 ka as in the case of Marsyandivalley.The terrace T2 as well as the fan lobe F2 of Rangpo

Khola and Ben Khola are composed mostly of phyllite andquartzite clasts of the Daling Formation present to thesouth of MCT. The angularity of the clasts indicates thatthey have not traveled a long distance as compared to thegravels in T3. The hyperconcentrated flow deposits (30%by volume) at the bottom of T2 capped by thick channelflow deposits (60% by volume) appear to have beenbrought into the trunk stream through the tributarychannels probably during a period of landslide activityalong these streams. Evidence of landslide activity in thepast exists along the south bank of Rangpo Khola in theform of landslide cones and rock falls. The facts that (i) thebase of the F2 fan lobe at Rangpo Khola contains ahyperconcentrated flow unit followed by a sequence ofchannel flow units (Fig. 6, litholog 11) and (ii) the basalunit of the same fan lobe at Ben Khola is a debris flowdeposit (Fig. 6, litholog 1) indicate that the landslide/bankfailure along the tributary streams supplied the material todevelop the terrace T2 and fan lobe F2. Similarly, the upperdebris flow facies E (Fig. 6, litholog 7) in T2 is probably theresult of slope failure along the Teesta River channelbetween the two tributary junctions. The predominance ofrounded granite gneiss boulders in terrace T2 on the eastbank of Teesta indicates that these materials were flowingin the trunk stream during the period when landslide bornematerials (phyllite and quartzite) were flushed through thetributary streams. Similar mass flow deposits (�4 ka) in theMarsyandi river valley have been attributed to a singlemassive landslide by Pratt-Sitaula et al. (2003). Suchlandslides could have been triggered by seismic events,and the deposits may not be indicative of longer-termforcing such as climate change. However, this aggradation


phase may have been followed by an incision phase due tomonsoon-controlled increase in discharge in the Mid-lateHolocene (�7 ka to present). In this period, an incision of8–12m (Table 1) would require high discharge, suggestinga stage in which sediment supply did not exceed the river’scarrying capacity.

At the west bank of the Teesta River, the base of T1 ischaracterized by channel flow deposits which are capped byhyperconcentrated flow deposits (Figs. 7c and 8). Itappears that the basal channel flow unit was formed bythe deposition of sediments carried by the main riverchannel whilst a later slope failure along the east bank ofthe Teesta contributed to an increase in the sediment fluxand its subsequent deposition as hyperconcentrated flow.This aggradation phase probably occurred in late Holo-cene. The hyperconcentrated flow in the present day Teestachannel is the result of bank failure and reworking ofsediment from earlier landforms due to immense dischargeduring the monsoon season. This aggraded valley wassubsequently incised up to 345m a.s.l.

Tectonics may have a strong influence on sedimentproduction in a basin. Uplift and slope failure may enhancesediment supply to a fluvial system, but these factorscannot increase water supply (Goodbred, 2003). In aregime of significant tectonic activity in a basin, hugesediment load can be generated, but these sediments cannotbe dispersed downstream without an increase in streampower. A 20–30m incision registered in the depositionallandform sequences along the northern and southerntributaries of the Ganga (Goodbred, 2003) as well as alongthe Teesta (considered to be a major tributary of the Gangain the historical past) indicates climatic influence onincision.

Further, rapid baselevel lowering produces a majorshort- lived pulse of sediment (Schumm et al., 1987; Gelliset al., 1991). The sediment production rate is reduced, andis delayed when the baselevel change is slow. Compared tobase level change, the effects of uplift in the sedimentsource region are very pronounced and longer-lasting withan exponential increase of sediment production asrelief increases (Schumm, 1963). However, the magnitudeof this uplift is not believed to be sufficient for lateQuaternary incision (cf. Molnar and England, 1990).Moreover, the dominance of competent rocks in thecatchments and vegetative cover would inhibit erosionthereby reducing the sediment production rate. Thepaucity of weak phyllite and quartzite boulders (theprovenance of which surrounds the study area) in theterrace T3 and dominance of well rounded granite gneissboulders suggests that processes other than those related tolocal tectonic effects or baselevel change supplied thesesediments.

In a climate-driven fluvial system a change from humidto semiarid (or to markedly seasonal climate) wouldproduce the greatest increase of sediment whilst the changefrom humid to arid would greatly decrease sedimentdelivery and the change from humid to sub- humid or

super-humid would have little effect on the production ofsediment flux (Langbein and Schumm, 1958; Schumm,1993). Climate changes will be rapid in comparison to mostof the baselevel and tectonic changes, and therefore, theresponse will be rapid (Schumm, 1993).Slope instability over extended periods is maintained by

active tectonics. However, precipitation is definitely one ofthe major triggering factors in Himalayan landslides.Present day landslide activity attains its maxima duringthe period when summer monsoon dominates in theHimalayan region. Higher precipitation is believed to becoupled with increased mass wasting and significantlyhigher sediment flux during intensified monsoon phases inwestern Himalaya (Bookhagen et al., 2005). The landslideactivity, supposed to have acted as a major contributor ofsediments to terrace T2 as well as fan lobes F2 and F1,could have been triggered by intense precipitation on thevalley slopes. This implies that rhythmicity in climate, evenat a shorter time scale, has played a major role in valleyalluviation in the Teesta. Furthermore, the spatial correla-tion of heavy rainfall and active deformation in Marsyandivalley in Nepal, about 400 km away, suggests thatprecipitation patterns may strongly combine with activefaulting and govern the geomorphology of the river system(Hodges et al., 2004). Moreover, a sufficient amount ofwater is required for the large volume of sediments to becarried down stream as debris, hyperconcentrated andchannel flows. This water would become available duringperiods of intense monsoon precipitation and/or melting ofice during interstadial periods. Thus, it appears that lateQuaternary climate change, particularly on a millennial tomulti-millennial scale, had a strong system-wide influenceon sediment production, transport and deposition in theTeesta River system.The interrelationship between climate, erosion, deposi-

tion and tectonic activities is not properly understood todate. However, it appears that major alluviation andincision events could be ascribed to the factors associatedwith climatic processes such as strengthening or weakeningof monsoonal precipitation and related fluvial discharge.Tectonic activity affects sediment fluxes and is responsiblefor the insetting of younger terraces/fanlobes into the olderterraces/fanlobes. During seismic events, landslide activityalong the slopes of river valleys influences sedimentdelivery into the valleys, causing the effects of tectonicsto be intricately coupled with that of climate.

Acknowledgements

We acknowledge the financial support from the NationalHydro-electric Power Corporation (NHPC), India, sanc-tioned to MKP and AB vide project no. J.12011/11/99-IA.I. We thank our colleagues Drs. M.S. Bisht, J.P. Bhatt,and D.C. Nautiyal as well as Mr. R. Mehta and Mr. S.Ghetodi for their help. We appreciate discussions withProfessor Martin Gibling. We express gratitude to Pro-fessor Steven L. Goodbred Jr. and Dr Malay Mukul for


their constructive and helpful reviews. Special thanks go toProfessor Rajiv Sinha for some important suggestions.

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